The present invention relates to a lower vehicle-body structure of a vehicle, and in particular to the lower vehicle-body structure of the vehicle in which a suspension cross member is fixed by fastening a bolt member to a nut member vertically fixed to a bottom plate portion of a support member.
Conventionally, a pair of right-and-left front side frames provided at a front vehicle body of the vehicle are connected to a front-face side of a dash panel, and rear end portions of the front side frames extend rearward along a slant portion of the dash panel to the vicinity of a front end of a floor panel and are joined to the dash panel and the floor panel from a front side of a cabin. Herein, the front side frames are configured such that their midway portions are bent in a vehicle width direction in front of the dash panel in order to reduce a collision load for a passenger in a vehicle frontal collision, thereby reducing collision energy of the collision load through bending deformation of the front side frames.
Further, a suspension cross member to support suspension arms for a front wheel is arranged in back of and right below a power unit. A lower vehicle-body structure of a vehicle disclosed in Japanese Patent Laid-Open Publication No. 2011-162159 is configured such that the suspension cross member extending in the vehicle width direction is provided in back of and right below the power unit which is arranged between a pair of right-and-left front side frames extending forward from a front portion of a cabin, a rear end portion of the suspension cross member is supported at a support portion which is provided at a side member connecting to a front end portion of a tunnel frame, a midway portion of the suspension cross member is supported at the front side frame via a middle attaching member extending upward, and a lower portion of a pipe-shaped attaching member which forms an upper end side portion of the middle attaching member is supported, having a lower support rigidity, in the longitudinal direction, than an inward portion, in the vehicle width direction, a front portion, and a rear portion of a side face portion.
As shown in FIG. 9, a rear-end side support portion of a suspension cross member 54 disclosed in the above-described patent document is provided with a reinforcing member 52 and a weld nut 53 inserted into the reinforcing member which are both arranged in a closed cross section formed by a dash panel 50 and a side member 51, and a bolt 55 inserted through a rear end portion of the suspension cross member 54 is fastened to the weld nut 53. When the suspension cross member 54 is moved back in a vehicle frontal collision, rotational movement is generated at the bolt 55 so that a lower end of the bolt 55 can be rotated rearward and upward (counterclockwise). Herein, a front-side lower end portion of the weld nut 53 is pressed against a front-side peripheral area of a bolt hole formed at a bottom plate portion 51a of the side member 51, which causes local stress concentration so that the bolt hole may be deformed, enlarged, or tear off. Consequently, the weld nut 53 may be pulled off the side member 51, whereby the rear end portion of the suspension cross member 54 may be detached from a vehicle body.
In a case where the performance of absorbing the collision energy by the front side frame is high, it is necessary to promptly detach the suspension cross member from the vehicle body in the vehicle frontal collision so that the collision-energy absorption performance of the front side frame can be maintained properly. However, there is room for further improvement in attaining the collision-energy absorption more stably.